Method of manufacturing magnetic heads having very short gap lengths



Feb. 8, 1966 s. DUINKER ET AL METHOD OF MANUFACTURING MAGNETIC HEADSHAVING VERY SHORT GAP LENGTHS Filed Feb. 26, 1962 smow DUINKERINVENTORJULES OS MABRJINBS J J SCHOONES I? AGENT United States Patent 3,233,308METHOD OF MANUFAQTURING MAGNETIC HEADS HAVlNG VERY SHORT GAP LENGTHSSimon Duinker, Jules Bus, and Martinus Josephus .lo-

hannes Schoones, Eindhoven, Netherlands, assignors to North AmericanPhilips (Iompany, Inc., New York, N.Y., a corporation of Delaware FiledFeb. 26, 1962, Ser. No. 175,536 Claims priority, applicationNetherlands, Mar. 30, 1.961, 263,099 3 Claims. (Cl. 29155.5)

This invention relates to a method of manufacturing magnetic heads. Moreparticularly, it relates to a method of manufacture of such heads whichhave very short gap lengths, the method providing a head which ismechanically strong and magnetically efficient.

The recording density required of some magnetic recordings has becomesuch that they can only be recorded and reproduced with the aid of headshaving very short gap lengths, for example from 1 to 2 microns. Toenable the gap walls to resist the forces occurring during the processof shaping the pole face of the head by grinding and polishing, theirdepth, corresponding to the height of the gap, has to be at least 150microns. If both walls consist of a material having a high magneticreluctance which is generally used in manufacturing heads of the typeunder consideration, for example ferrite, the efiiciency of the head isappreciably lower than that of heads having longer gap lengths. This isdue to the fact that in the head having a short but comparatively deepgap the magnetic reluctance of the useful path of the lines of magneticflux, i.e., the path through the magnetic record carrier, is largecompared with the magnetic reluctance of the gap. To obtain asatisfactory efficiency with a gap length of from 1 to 2 microns the gapheight consequently has to be limited to about microns; this value ofgap height, however, is only a fraction of the value required in orderto give the head sufiicient mechanical strength, the latter value beingat least 150 microns.

It is the object of the present invention to provide a magnetic headhaving a very short gap length which is mechanically strong andmagnetically eflicient. In accordance with one aspect of the invention,one of the two plates from which the pole pieces of the head are made isscored by grinding for part of the surface by which it engages the otherplate; the depth of scoring appreciably exceeds the desired gap lengthof the finished head, and the recess formed by scoring is filled with astrengthening material having a comparatively high melting point, forexample glass, after which the said surface is ground and polished;after the other plate has been ground and polished, it is pressedagainst the first plate with the interposition of a thin layer ofnon-magnetic filling material having a lower melting point than thestrengthening material and having a thickness equal to the desired gaplength; finally the poleface of the head is given the desired outlineand the gap height is given the desired value by grinding and polishing.As used herein, the gap length is to be understood as the dimensionindicated in the drawing; the gap height is the dimension of the gap inthe vertical direction of the plane of the drawing. This dimensionalnomenclature is that employed and shown in an article by S. Duinker,Durable High-Resolution Ferrite Transducer Heads Employing Bonding GlassSpacers in Philips Research Reports, vol. 15, 1960, pages 342-367; seeespecially FIG. 1 of this article.

The above mentioned recess provided by grinding the surface of one plateis preferably shaped so as to be rounded at the end nearer the futurepoleface of the head, the radius of the rounded end preferably being afew times greater than that corresponding to the depth of the recess inthe said surface.

In order that the invention may readily be carried into effect, it willnow be described more fully, by way of example, with reference to theaccompanying drawing in which the single figure is a cross-sectionalview to an enlarged scale of two plates of ferrite material preparedwith the use of the invention and'secured to one another.

The figure shows two ferrite plates 1 and 2, the lefthand plate 1 beingrecessed as shown generally by reference numeral 4 by grinding to adepth of about microns. At the end adjacent the future poleface of thehead, which is shown by a broken line 3, this recess 4 terminates in arounded part 5 through a distance exceeding the desired gap height, forexample through a distance of from 100 to 200 microns. The rounded part5 serves to limit the magnetic reluctance of the flux lines traversingthe gap 9 to a value which is not unnecessarily large.

Subsequently the recess 4 is filled with a layer 6 of glass or othernon-magnetic strengthening material having a high melting point. Thesurface of the plate 1 to be engaged by the plate 2, which surfaceconsequently largely consists of glass, now is ground fiat and polishedand then coated with a film It of enamel or other suitable fillingmaterial for the future gap having substantially the thickness of thedesired gap length, the melting point of this material being lower thanthat of the above mentioned strengthening material.

After the plate 2 has been ground flat and polished it is pressedagainst the plate 1 while heating until it firmly adheres thereto. Thepressure employed is not so great as to force any appreciable amount ofthe gap forming material from between the plates. The poleface of thehead is then given the desired outline and the gap is given the desiredheight of, for example, 25 microns by grinding and polishing, duringwhich process the glass or other strengthening material 6 prevents thetapering part of the plate 1 from collapsing under the mechanicalpressure inevitable in grinding and polishing. Owing to its high meltingpoint the glass 6 retains the required strength during this operation.In order to prevent the gap edges from being chipped during thisoperation, the coetficient of expansion of the enamel in the gap 9 hasto be matched to that of the ferrite.

Finally the plates may be firmly secured, with the poleface down, in ajig which is shaped in accordance with the pole-face, after which withthe aid of asuitable grinding tool capable of being lowered in the planeof the gap so much material is removed by grinding as to produce theoutline shown in the figure. It should be ensured that at the gap end apart of the plate 2 of at least microns high is left and that at thispoint the two plates are kept together by the portion 6 of thestrengthening material which is not removed by grinding. The polefaceoutlines may be produced by any convenient method and they mayalternatively be produced before the parts 1 and 2 are secured to oneanother. Between the extending legs 7 and 8 of the plates an amount ofspace is thus produced sufiicient to accommodate the windings of thehead.

In the foregoing, the invention has been described solely in connectionwith a specific embodiment. Many modifications in the method will bereadily obvious to those skilled in the art without departing from thespirit of the inventive concept, the scope of which is set forth in theappended claims.

What is claimed is:

l. A method of manufacturing a magnetic head having a very short gaplength and composed of at least two magnetic plates forming the headpole pieces each having a surface and an adjoining edge, comprising:grinding a recess in the part of the edge of one of said plates which isadapted to confront the edge of the other plate to form the gap, saidrecess being ground to a depth substantially exceeding the desired gaplength of the completed head and being located at a portion of saidplate remote from where the surface and edge are joined, said grindingleaving intact a conjoining portion of said edge where the surface andedge are joined, filling said recess with a non-magnetic strengtheingmaterial having a comparatively high melting point, said filling beingdone at said high melting point such that said strengthening material isfusion bonded to the plate and has an exposed part in approximatealignment with said conjoining portion, grinding and polishing saidconjoining part of said one edge and said exposed part of thestrengthening material, grinding and polishing said edge of said otherplate, interposing a thin layer of gap filler material between saidexposed part and conjoining portion of said one plate and said edge ofthe other plate, and pressing them together, said gap filler materialhaving a comparatively low melting point and having a thicknesssubstantially equal to the desired gap length, said pressing being doneat said low melting point such that the parts which are pressed togetherare fusion bonded to each other and thereby leaving a portion of saidthin layer thus forming the gap of the head, and grinding the surfacesof the plates in the vicinity of the gap to achieve the desired polefaceoutline and the desired gap height.

2. A method of manufacturing a magnetic head having a very short gaplength and composed of at least two magnetic plates forming the headpole pieces each having a surface and an adjoining edge, comprising:grinding a rounded recess in the part of the edge of one of said plateswhich is adapted to confront the edge of the other plate to form thegap, said recess being ground to a depth substantially exceeding thedesired gap length of the completed head and being located at a portionof said plate remote from where the surface and edge are joined, saidgrinding leaving intact a conjoining portion of said edge where thesurface and edge are joined and being rounded at the portion remote fromthe edge, filling said recess with a non-magnetic strengthening materialhaving a comparatively high melting point, said filling being done atsaid high melting point such that said strengthening material is fusionbonded to the plate and has an exposed part in approximate alignmentwith said conjoining portion, grinding and polishing said conjoiningpart of said one edge and said exposed part of the strengtheningmaterial, grinding and polishing said edge of said other plate,interposing a thin layer of gap filler material between said exposedpart and conjoining portion of said one plate and said edge of the otherplate, and pressing them together, said gap filler material having acomparatively low melting point and having a thickness substantiallyequal to the desired gap length, said pressing being done at said lowmelting point such that the parts which are pressed together are fusionbonded to each other and thereby leaving a portion of said thin layerthus forming the gap of the head, and grinding the surfaces of theplates in the vicinity of the gap to achieve the desired polefaceoutline and the desired gap height.

3. A method of manufacturing a magnetic head having a very short gaplength and composed of at least two ferrite plates forming the head polepieces each having a surface and an adjoining edge, comprising: grindinga rounded recess in the part of the edge of one of said plates which isadapted to confront the edge of the other plate to form the gap, saidrecess being ground to a depth substantially exceeding the desired gaplength of the completed head and being located at a portion of saidplate remote from where the surface and edge are joined, said grindingleaving intact a conjoining portion of said edge where the surface andedge are joined and being rounded at the portion remote from the edge,filling said recess with glass having a comparatively high meltingpoint, said filling being done at said high melting point such that saidglass is fusion bonded to the plate and has an exposed part inapproximate alignment with said conjoining portion, grinding andpolishing said conjoining part of said one edge and said exposed part ofthe glass, grinding and polishing said edge of said other plate,interposing a thin layer of gap filler material between said exposedpart and conjoining portion of said one plate and said edge of the otherplate, and pressing them together, said gap filler material having acomparatively low melting point and having a thickness substantiallyequal to the desired gap length, said pressing being done at said lowmelting point such that the parts which are pressed together are fusionbonded to each other and thereby leaving a portion of said thin layerthus forming the gap of the head, grinding the surfaces or" the platesin the vicinity of the gap to achieve the desired poleface outline andthe desired gap height, and grinding away a portion of said plates andsaid glass at a point remote from the surface.

References Ziterl by the Examiner UNITED STATES PATENTS 2,915,81212/1959 Rettinger 29-15559 2,933,565 4/1960 Neumann 179-1002 3,079,4702/1963 Camaras 179-1002 3,094,772 6/1963 Duinker 29-15556 3,104,4559/1963 Frost 29-1555 FOREIGN PATENTS 1,031,020 5/ 1958 Germany.

827,335 2/1960 Great Britain.

852,533 10/1960 Great Britain.

WHITMORE A. WILTZ, Primary Examiner.

LESTER M. SWINGLE, JQHN F. CAMPBELL,

Examiners,

1. A METHOD OF MANUFACTURING A MAGNETIC HEAD HAVING A VERY SHORT GAPLENGTH AND COMPRESSED OF AT LEAST TWO MAGNETIC PATES FORMING THE HEADPOLE PIECES EACH HAVING A SURFACE AND AN ADJOINING EDGE, COMPRISING:GRINDING A RECESS IN THE PART OF THE EDGE OF ONE OF SAID PLATES WHICH ISADAPTED TO CONFRONT THE EDGE OF THE OTHER PLATE TO FORM THE GAP, SAIDRECESS BEING GROUND TO A DEPTH SUBSTANTIALLY EXCEEDING THE DESIRED GAPLENGTH OF THE COMPLETED HEAT AND BEING LOCATED AT A PORTION OF SAIDPLATE REMOTE FROM WHERE THE SURFACE AND EDGE ARE JOINED, SAID GRINDINGLEAVING INTACT A CONJOINING PORTION OF SAID EDGE WHERE THE SURFCE ANDEDGE ARE JOINED, FILLING SAID RECESS WITH A NON-MAGNETIC STRENGTHENINGMATERAL HAVING A COMPARATIVELY HIGH MELTING POINT, SAID FILLING BEINGDONE AT SAID HIGH MELTING POINT SUCH THAT SID STRENGTHENING MATERIAL ISFUSION BONDED TO THE PLATE AND HAS AN EXPOSED PART IN APPROXIMATEALIGNMENT WITH SAID CONJOINING PORTION, GRINDING AND POLISHING SAIDCONJOINING PART OF SAID ONE EDGE AND SAID EXPOSED PART OF THESTRENGTHENING MATERIAL, GRINDING AND POLISHING SAID EDGE OFF SAID OTHERPLATE, INTERPOSING A THIN LAYER OF GAP FILLER MATERIAL BETWEEN SAIDEXPOSED PART AND CONJOINING PORTION OF SAID ONE PLATE AND SAID EDGE OFTHE OTHER PLATE, AND PRESSING THEM TOGETHER SAID GAP FILLER MATERIALHAVING A COMPARATIVELY LOW MELTING POINT AND HAVING A THICKNESSSUBSTANTIALLY EQUAL TO THE DESIRED GAP LENGTH, SAID PRESSING BEING DONEAT SAID LOW MELTING POINT SUCH THAT THE PARTS WHICH ARE PRESSED TOGETHERARE FUSION BONDED TO EACH OTHER AND THEREBY LEAVING A PORTION OF SAIDTHIN LAYER THUS FORMING THE GAP OF THE HEAD, AND GRINDING THE SURFACESOF THE PLATES IN THE VICINITY OF THE GAP TO ACHIEVE THE DESIRED POLEFACEOUTLINE AND THE DESIRED GAP HEIGHT.